High temperature composite projectile barrel

ABSTRACT

A composite projectile barrel is disclosed comprising a polymer matrix composite outer shell that accommodates higher temperature loading. In one embodiment, the invention comprises an outer shell fabricated from carbon fibers and polyimide resin having a glass transition temperature greater than 500° F. In another embodiment, the resin mixture includes a plurality of sizes of aluminum particles, between about 0.1 microns and 10.0 microns in diameter and of approximately spherical shape, as a thermal conductive additive.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to two Provisional patent applicationNo. 61/871,154 filed Aug. 28, 2013 and No. 61/873,771 filed Sep. 4,2013. The entire disclosures of both provisional applications are herebyincorporated by reference and relied upon.

BACKGROUND OF THE INVENTION

Users have long desired lighter gun systems that remain durable andreliable. It is known to substitute relatively strong but lightweightmaterials—such as unreinforced and reinforced polymers, continuous glassfiber or carbon fiber composites—for various portions of the guncommonly fabricated from steel, aluminum, or other metals. Attention hasfocused on gun barrels, which constitute a large percentage of a gun'sweight. It is known, for example, to fabricate a gun barrel having asteel inner liner surrounded by a carbon fiber reinforced polymer matrixcomposite (PMC) outer shell, incorporating a resin. This combinationlightens the gun while retaining good barrel strength and stiffness.

The carbon fibers used in the PMC outer shell may be any type thatprovides the desired stiffness, strength and thermal conductivity.Typically for PMC gun barrel applications, polyacrylonitrile (“PAN”)precursor or pitch precursor carbon fibers are used. The carbon fibermay be applied as dry carbon fiber strands or tows which are combinedwith a resin in a “wet” dip pan process, then wound around the innerliner. Alternatively, the shell may be built from carbon fiber tow,unidirectional tape, or fabric that was previously impregnated withresin in a separate process (“towpreg” or “prepreg”), then applied tothe inner liner. Whether applied wet or dry, the matrix resin istypically an epoxy. The composite barrel may then be cured, finished,and attached to a receiver and stock. Such carbon fiber/epoxy resinmatrix composites can provide a suitable balance of thermal properties,mechanical properties, and processing characteristics for many commonfirearms applications.

When a composite barrel is subjected to high heat from rapid orprolonged firing, however, they are usually less durable than solidsteel barrels. Temperatures within the barrel of a semi-automatic or anautomatic rifle, for example, can easily exceed 500° F., and may exceed700° F. or higher. Firearms made with barrels manufactured entirely fromsteel and similar materials are durable and have a sufficiently highheat transfer characteristics to dissipate heat quickly enough toaccommodate such firing applications and still perform acceptably.Existing gun barrel composite shells have lower thermal conductivity inthe radial direction than steel, such that the composite materialeffectively acts like a heat insulator. Types of steel typically used ingun barrels have a thermal conductivity of about 20-40watts/meter-Kelvin (20-40 W/m·K). A typical PAN carbon fiber epoxycomposite has a thermal conductivity of only about 0.5 W/m·K in the“through thickness” direction, or radial direction in a gun barrelapplication. Typical values for the “in plane” (the fiber direction) forthese composites are on the order of only 1-5 W/m·K. As discussed below,PMC materials also degrade at lower temperatures than steel.

U.S. Pat. No. 6,889,464 (Degerness) disclosed a gun barrel comprising asteel inner liner helically wound with carbon fiber filaments, or tows,drawn through a wet epoxy resin mixture in a dip pan bath process.Degerness added thermally conductive material to the epoxy resincomprising chopped/milled pitch carbon fibers, commercially available asThermalgraph®. The cured barrel exhibited significantly higher thermalconductivity and improved heat dissipation from the inner barrel throughthe PMC to the ambient atmosphere. Even with the addition of thermallyconductive chopped carbon pitch, however, rapidly firing a gun fittedwith a carbon fiber composite barrel may cause barrel temperatures tosignificantly exceed the use-temperature capability of epoxy resins. Asthe barrel heats due to prolonged firing, it can exceed the glasstransition temperature, T_(g), of the cured epoxy-based resin mixture.At the T_(g), the PMC softens significantly and the mechanical integrityof the composite barrel is compromised. As the barrel is heated to evenhigher temperatures irreversible thermal decomposition of the curedepoxy matrix occurs and barrel structural integrity is furthercompromised. Epoxy resins having desirable processing characteristicsand cured thermal and mechanical properties typically exhibit a glasstransition temperature in the range of 140-400° F., and typicallyexhibit thermal decomposition at temperatures above 500° F.

Theoretically, it should be possible to improve thermal conductivity inthe PMC and retard heat buildup by increasing the proportion ofThermalgraph or of other thermally conductive material(s) in the resinmixture, such as graphene, graphite, carbon nanotubes, ceramic particlesor metal particles. The desirable characteristics for a thermallyconductive additive are high thermal conductivity, low density, and ofappropriate size and size distribution to occupy spaces between thereinforcing fibers. Because all of these thermally conductive additivestend to strongly increase the viscosity of the resin, however, higherconcentrations of the thermally conductive additive make the resinmixture more viscous, inhibiting complete coating of the carbon fibertow with resin and making manufacturing more difficult and lessconsistent. Additionally, high loadings of conductive additivesgenerally diminish the mechanical properties (e.g., strength) of thecomposite.

Other resins having higher glass transition temperatures than epoxyexist, but they are generally more difficult to process and manufacturePMC articles with and are significantly more expensive than epoxies. Itis known that resins having polyimide chemistry have significantlyhigher glass transition temperatures, better thermal conductivity, andimproved thermal stability in comparison with epoxy resins. Althoughcured polyimide resins have superior thermal performance as compared toepoxy resins, many have relatively high toxicity from the solvents andmonomers used in their manufacture. In general, the thermoset class ofpolyimide resins that comprise reactive monomers in a solvent are knownas “Polymerizable Monomer Reactant” or PMR polyimides. A claimed lowertoxicity polyimide resin was disclosed in U.S. Pat. No. 5,171,822(Pater) “Low Toxicity High Temperature PMR Polyimide,” and iscommercially available under the name RP46. The RP46 resin, however, athigh enough solids concentration for manufacturing PMCs is semi-solid atroom temperature; its high viscosity makes it very difficult to workwith when “wet” winding fiber filament tows. A PMR polyimide resinhaving a higher glass transition temperature than epoxy and usefulprocessing characteristics was disclosed in U.S. Pat. No. 6,889,464(Lincoln). A resin incorporating the Lincoln chemistry is manufacturedby Performance Polymer Solutions, Inc., 2711 Lance Drive, Moraine, Ohio45409, under the name P²SI® 635LM. The P²SI 635LM resin has a suitablyhigh glass transition temperature of 635° F., but is also significantlymore viscous at room temperature than typical wet filament winding epoxyresins, seemingly precluding its use in filament winding applications,such as when attempting to wind resin-infused fiber tows in multiplelayers upon an inner liner of a gun barrel.

Another obstacle to using polyimide resins such as the P²SI® 635LM resinin filament winding applications relates to processing difficulties tocure a freshly-wound barrel. When curing polyimide matrix carbon fibercomposite wound barrel comprising a solvated polyimide resin, volatilesin the resin are released. When curing flat or large radius panels,these gasses more easily migrate to the surface or edges of the panel,making it easier to produce a composite article substantially free ofundesirable voids. When curing a wound gun barrel that inherently hasminimal part “edges”, however, it has proven difficult to remove thevolatile products and gasses because they tend to become trapped betweenthe continual filament windings. Unlike curing a panel having largeplanar surfaces, the migration path for volatile gasses released from afreshly wound polyimide resin composite gun barrel is in the radialdirection outward through the thickness of the composite (with somelongitudinal migration). This problem is compounded when utilizing thehigher viscosity polyimide resins. Despite the advantages of using apolyimide resin to wrap a thin cylinder such as a small caliber gunbarrel, use of polyimide resins for PMCs—even with alternative compositemanufacturing techniques such prepreg, towpreg, and resin transferinfusion—has been substantially confined to flat or relatively largeradius carbon fiber sheets or panels.

Another resin class having a relatively high glass transitiontemperature is polyetheretherketone (PEEK). PEEK is significantly moreexpensive than epoxies. Further, the glass transition temperature ofPEEK is only about 290° F., with (even more costly) higher-temperatureformulations exhibiting glass transition at about 315° F. These glasstransition temperatures are still lower than desirable in a rapid-fireweapon. PEEK is also a thermoplastic material, meaning it is a highmolecular weight polymer. It is processed in a melted state at atemperature well above its T_(g) (typically at ˜700° F.) to achieve flowand consolidation, thus it is applied hot and cools to a rigidstructural state. Typical epoxies and PMR polyimides, on the other hand,are thermoset materials, meaning they are typically applied as lowviscosity monomeric resins at or near room temperature and aresubsequently cured for a period of time, often under elevatedtemperature and/or pressure conditions to form a densely cross-linkedhigh molecular weight structural material. Thermoplastic resins such asPEEK require very different manufacturing techniques and equipment thanthermoset resins.

Thermal conduction within the PMC tends to be strongly affected by theorientation of fiber, being higher in the longitudinal direction of afilament than transversely across. Depending on which specific carbonfiber is used, thermal conductivity of a PAN fiber, for example, couldbe higher than steel (about 20-40 W/m·K) in the longitudinal direction,but less than 10 W/m·K in the transverse or radial direction. The resinbetween the fibers in the cured PMC is even less thermally conductivethan the transverse thermal conductivity of the fibers. When a PMC isused as an outer shell for a barrel, solving the heat problem isdifficult because most heat must be conducted radially to the outsidesurface of the barrel and ambient atmosphere, through the compositeshell, requiring heat to transfer through resin and transversely acrossindividual fibers.

Thermal conductivity is affected not only by the type of resin, fibers,and any additive(s) and their relative proportions, but also by the sizeof the fibers and size of additive particles. For example, a typicalindividual PAN carbon fiber might have a diameter between about 5 and 10microns. A carbon fiber filament tow comprises a plurality of fibers,with a tow typically used for a gun barrel application having about6,000 to 24,000 individual fibers. After the resin-coated carbon fibertow is wound around the barrel and cured, the resin will bond all thecontinuous reinforcing fibers together to provide mechanical integrityand durability. Ideally the cured resin will fill essentially all of thespace between the individual carbon fibers. The volume or space betweenthe individual carbon fibers—referred to as the “unoccupied volume orspace,” the “inter-fiber volume,” or the “interstitial space”interchangeably—is thus ideally occupied by cured resin in the PMC. Atthe micro level (i.e., the scale of the fiber diameter, approximately 10microns), significant obstacles to transferring heat from the hot steelinner barrel through the PMC are the lower thermal conductivity of theresin between the fibers, heat transfer resistance at the polymermatrix-fiber interface, and heat transfer resistance at the polymeradditive particle interface.

The effect of the thermally conductive filler particle on resinviscosity depends on many factors such as size, size distribution,shape, and interactions of the particles with each other and the resin.Ideally, an effective presence of the thermally conductive additivewould be uniformly dispersed with the resin mixture throughout theinterstitial space. However, the interstitial space is not uniform; thespace may range from, for example, from about <1 micron to about 50microns. Therefore, effective quantities of particles sized small enoughto occupy the smaller interstitial spaces will tend to make the resinmixture too viscous, and may locally weaken the matrix if/where thesmaller additive particles “clump” in the larger interstitial spaces. Onthe other hand, if the thermally conductive material particles are sizedtoo large, larger than available interstitial spaces, they will not fitinto the smaller interstitial spaces, thus displacing the continuousreinforcing fibers. This results in lower composite fiber volumefraction, compromised mechanical properties, and lower thermalconductivity throughout the PMC.

What is needed is a PMC resin mixture for the outer shell of a compositegun barrel that is workable for manufacturing, that when curedwithstands high operating temperatures and/or more effectively transfersto the ambient atmosphere the heat generated by rapid or prolongedfiring of a gun, and that is light, stiff and strong.

BRIEF SUMMARY OF THE INVENTION

A composite projectile barrel is disclosed comprising a novel polymermatrix composite outer shell that accommodates higher temperatureoperation. In one embodiment, the invention comprises a barrel fordirecting the path of a dischargeable projectile including an innerliner defining an axial bore and an outer shell surrounding and indirect contact with the inner liner, said outer shell fabricated from apolymer matrix composite comprising a resin mixture and fibers, withsaid resin mixture having a fully cured glass transition temperature,T_(g), greater than 500° F. In another embodiment, heat conductionthrough the PMC is facilitated by including a thermal conductiveadditive in the resin mixture, comprising generally spherical metallicparticles between about 0.1 microns and 10.0 microns in diameter.

It is to be understood that the invention may be practiced with manymakes and models of projectile barrels with comparable effectiveness,and on other structures where fiber is combined with a resin and woundor otherwise constructed around along an elongated axis.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more readily appreciated when considered in connection with thefollowing detailed description and appended drawings, wherein:

FIG. 1 illustrates a rifle fitted with a composite barrel;

FIG. 1A is a cut-away of a portion of the composite barrel shown in FIG.1;

FIG. 2 illustrates a resin tow winding system;

FIG. 3 illustrates a dry towpreg winding system;

FIG. 4 is a cut-away illustration showing an embodiment of a compositebarrel; and

FIG. 5 is a greatly magnified view of a sectional cut through anexemplar polymer matrix composite.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures, wherein like numerals indicate like orcorresponding parts throughout the several views, FIG. 1 shows a boltaction rifle 10 fitted with a receiver 12, stock, trigger, barrel 14,and other familiar features. In the embodiment shown, barrel 14 securelyengages with receiver 12 by means of threads 16. In operation, acartridge of ammunition is inserted into the receiver. The cartridge hasa base portion containing a gunpowder charge and dischargeableprojectile, i.e., a bullet. When a shooter pulls the trigger, a firingpin strikes the base of the cartridge, igniting the gunpowder charge andcausing the bullet to discharge through axial bore 24 and out of themuzzle 18.

As shown in FIG. 1A, barrel 14 is comprised of an inner liner 22 and anouter shell 20. In one embodiment, inner liner 22 is made of a metal,such as a steel alloy. A metal inner liner, such as stainless steel,facilitates fabrication of rifling lands and grooves along axial bore 24as well as threads at the muzzle and/or breech ends of the barrel. Theinner liner may also be a nonmetallic material such as a ceramic or apolymer-based material. Outer shell 20 is a cured polymer matrixcomposite (PMC) comprised of carbon fiber and a resin mixture asdescribed more fully below. Inner liner 22 need not be uniformlycylindrical. For example, inner liner 22 may radially expand at thebreech end to accommodate cutting of threads 16 for insertion intoreceiver 12, taper outwards at the muzzle 18, or include otherconfigurations depending on desired features of the gun. Outer shell 20likewise may include noncylindrical features or be discontinuous overthe length of barrel 14.

Outer shell 20 is in direct contact with inner liner 22 at interface 26.It may be desirable to promote adhesion or to inhibit corrosion betweenthe inner liner 22 and PMC outer shell 20 at interface 26. For purposesof this specification and the claims, “direct contact” means that theouter surface of inner liner 22 at interface 26 may include a surfacetreatment that is applied before outer shell 20 is fabricated upon innerliner 22. For example, a PMC outer shell 20 is in “direct contact” witha steel inner liner 22 at interface 26 even if the steel liner's surfaceis electroplated, anodized, or coated with a chemical compound ormixture, such as paint, resin or other substance.

FIG. 2 shows a simplified tow winding system 30 useful for fabricating acomposite gun barrel 14 having a PMC outer shell 20. In one embodiment,outer shell 20 comprises continuous fiber filament, or tow, 34, suppliedfrom tow spool 32. In another embodiment (not shown) the fiber could bein the form of fabric or a weave. Carbon fibers are typicallyadvantageous to use for PMC gun barrels due to their high stiffness,high strength, and low density. The term “carbon fiber” is used togenerically describe carbon and graphite fibers irrespective of theirmanufacturing process or precursor materials, and specifically includesboth PAN precursor and pitch precursor carbon fibers. In one embodiment,tow 34 is PAN carbon fiber filament tow, such as HexTow IM2A availablefrom Hexcel Corporation, Stamford Conn. However, tow 34 could also be apitch carbon fiber, such as GRANOC CN-60-A2S, available from NipponGraphite Fiber Corporation, Tokyo, Japan, or any suitable fiber formanufacturing composites including Kevlar, glass, quartz, ceramic,mineral, carbon, metallic, graphite, or hybridizations of fibers formedby combining different types of fibers to gain characteristics notattainable with a single reinforcing fiber.

Tow 34 is drawn from tow spool 32 under tension by rotating inner liner22 which functions as a mandrel. Inner liner 22 is placed between chucks47 and rotates about axial bore 24. The rotating inner liner 22 tugs tow34 through a resin mixture 36, dipping around a series of rollers 38immersed in resin bath 35, with the rollers 38 helping to press resinmixture 36 into tow 34. Those skilled in the art will appreciate thatthere are multiple ways of applying resin to the tow. In anotherembodiment (not shown), tow 34 could be drawn across the upper surfaceof a semi-immersed rotating drum wetted with resin.

Brisk movement of tow 34 through resin mixture 36 and around rollers 38creates currents and turbulence helping to maintain resin solids andother particulates in suspension within resin mixture 36. Optionally, anagitator (not shown) placed in resin bath 35 may be utilized tofacilitate uniform mix and viscosity of the resin, solvent, and anyadded particulates or other thermally conductive materials added assolids to the resin mixture 36. The agitator may be a mechanical paddledriven by a motor, a resin mixture recirculation system driven by apump, an ultrasonic agitator, or other means for maintaining solids andparticulates in suspension.

After the filament is impregnated with the resin mixture 36, excessresin mixture is removed from the tow. Excess resin mixture may beremoved from the tow by means of nip rollers 40 having an appropriategap setting, scrapers (not shown), appropriately-sized dies (not shown)and/or other means known in the art, individually or in combination.

Resin infused tow 42 exits resin bath 35 and is drawn through a filamentguide orifice 46 controlled by filament guide structure 44. Optionally,one or more heating elements 48 may flash off first stage volatilespresent in resin mixture 36 after the resin infused tow 42 exits resinbath 35 by means of a heat unit 48. The heating units causevolatilization of some or even most of any solvent that is present onresin infused tow 42. The heating elements 48 may be placed anywhere onthe path of resin infused tow 42, including heating the mandrel innerliner 22 itself. The heating elements may be radiant heaters, tubefurnace/heaters, convection heaters, or other means of heating resininfused tow 42, including various types of heating elements incombination.

After the excess resin mixture 36 is mechanically removed and optionallysubjected to heating, resin infused tow 42 is wound around the innerbarrel in the desired helical pattern and to a desired diameter.Filament guide structure 44 includes a mechanism for moving filamentguide orifice 46 in a lateral motion generally parallel to axial bore24, thereby guiding resin infused tow 42 back and forth along rotatinginner liner 22, so that resin infused tow 42 is applied to inner linerin a helical winding pattern. Filament guide orifice 46 itself may alsorotate or translate relative to filament guide structure 44.

It will be appreciated that if inner liner 22 rotates at a constantrate, faster lateral movement of filament guiding structure 44 willresult in a helical winding pattern of resin infused tow 42characterized by smaller winding angles relative to axial bore 24. At abrisk lateral speed, the helical winding angle of resin infused tow willbe small, nearly longitudinal relative to axial bore 24. Conversely,slower lateral movement of filament guiding structure 44 will result inlarger helical winding angles relative to axial bore 24. At very slowlateral speeds, winding angles of resin infused tow 42 may be nearlycircumferential hoops, almost 90 degrees. For purposes of the claims andthis specification, such nearly circumferential hoops are nevertheless“helical.” Tow winding system 30 may be controlled by a computerprocessor, so that rotation speed of the inner liner 22, lateralmovement of the filament guide structure 44, movements of filament guideorifice 46, tension applied to tow 34, and other aspects may beprogrammed by a user to produce desired patterns and sequences ofwinding angles, number of layers, and depths of the layers. Such systemsare available from, for example, McLean Anderson, 300 Ross Avenue,Schofield, Wis. 54476.

In a preferred embodiment, resin mixture 36 comprises a thermoset PMRpolyimide resin. However, currently available polyimide resins are tooviscous at room temperature, without excess solvent addition, to coattow 34 satisfactorily. Further, resin solids or other components such asparticulates may separate within resin mixture 36. Additional measures,such as heating or solvating resin mixture 36, are thus required toreduce viscosity and ensure satisfactory impregnation of tow 34.

Resin bath 35 may be configured to heat resin mixture 36 usingtechniques known to those skilled in the art, such as circulating a hotfluid, such as water, through a jacket surrounding resin bath 35, orapplying heating elements to the bottom or sides of resin bath 35, orvia a heating coil immersed in resin mixture 36. Resin mixture 36comprising a thermoset polyimide resin may be heated up to about 200°F., the precise temperature being dependent on the characteristics ofthe resin and the volatility of the solvent used, with somewhat lowertemperatures preferred. Higher temperatures make resin mixture 36 lessviscous, enabling better impregnation and more uniform winding, butaccelerate solvent loss and may accelerate premature cure reactions inthe polyimide resin (e.g., imidization) thereby reducing “pot life” ofthe resin.

Resin mixture 36 preferably comprises a solvent. Many solvents may beutilized to make the polyimide resin less viscous, including alcohols,aprotic solvents, and mixtures thereof. The PMR polyimide resin willtypically include an alcohol co-reactant that acts as a solvent. Asolvent having a lower boiling point (i.e., higher volatility) isgenerally more desirable because it can be more easily flashed off theresin infused tow 42 with heating units such as a heat unit 48. Methanoland ethanol are preferred solvents. The inventors have determined thatheating P²SI 635LM PMR polyimide resin mixture 36 to about 40° C. to 60°C. in resin bath 35, and adding methanol solvent to reduce the viscosityof resin mixture 36 to about 1000 cP, yields good resin impregnation anduniform filament winding operations. It is possible to achieve lowerviscosity and better handling characteristics by adding more solvent.However, too much solvent will result in insufficient resin solids inresin mixture 36 to adequately impregnate a carbon fiber tow 34 withresin. Using too high of a temperature to reduce the resin viscosityresults in undesirable side-reactions that reduce the cured thermal andmechanical properties of the polyimide polymer matrix.

A solvent such as methanol in resin mixture 36 has a lower boiling pointthan the polyimide resin. It is preferable to flash off much or most ofthe solvent on resin infused tow 42 before it is covered by subsequentwindings of tow. As discussed above, heating means may include one ormore radiant heaters 48, tube heaters, convective heaters, conductiveheat originating from a heated mandrel, or other heating means. In oneembodiment, a tube heater surrounds resin infused tow 42 and blows airheated to about 300° F. along the tow, directed back towards resin bath35, and a radiant heater directs heat upon rotating inner liner 22.

Rather than drawing a tow through wet resin, a dry towpreg (i.e., fiberthat has been previously coated and/or impregnated with a resin having ahigh glass transition temperature) may be wrapped on the rotating innerliner 22 then dry-cured with heat and/or pressure. Imidized towpreg maybe fabricated by first processing a polyimide resin to a partially-curedstate in the following manner. A polymerizable monomeric polyimide resinis heated to about 300-500° F. for between about 30 minutes to fourhours to imidize the resin so that oligomers form, having reactiveendcaps. Preferably, the heat is withdrawn and the resin is cooledbefore the functional endcapping agents on the oligomers commencesignificant reacting and cross linking. The imidized polyimide resin,being now in solid form, may then be ground into a fine powder. Thispowder may then be electrostatically coated on a fiber or split tape,then optionally thermally fused to the fiber or tape before re-spooling.

FIG. 3 shows a towpreg winding system similar to the tow winding systemof FIG. 2. A fiber spool 32 carries a supply of partially cured towpreg37 prepared as described immediately above. Instead of a resin bath, thetowpreg is heated prior to and/or during application to rotating innerliner 22 in order to soften the partially cured polyimide resinpreviously incorporated into towpreg 37 thus allowing it to flow andfacilitate consolidation. FIG. 3 shows several variants of possibleheating techniques, including a tube heater 39 and radiant heaters 41.Other heat sources include infrared heaters, hot air jets, and laserheating. Both the towpreg and the rotating inner liner 22 may be heatedabove the melting point of the polymer to achieve melt/melt contactbetween the towpreg and the inner liner. The rotating inner liner 22 maybe heated, for example, by a radiant heater 41 and/or by a cartridgeheater (not shown) placed within axial bore 24. Any of the foregoingheater types, alone or in combination, may be used to heat the towpregand/or inner liner so that the towpreg achieves good melt contact withthe inner liner. The softened and heated towpreg 43 is wound aroundinner liner 22 in a fashion similar to that described for the wet resintow winding system described above.

FIG. 4 shows an exemplar barrel 14 produced by the winding systemdescribed, comprising a PMC outer shell 20 progressively cut away toreveal a plurality of winding layers created by winding resin infusedtow 42 (or heated towpreg 43) around inner liner 22. In the embodimentillustrated, each winding layer has a different helical wrapping angle.First layer 50 has a first wrapping angle 58, second layer 52 has asecond wrapping angle 60, and third layer 54 has a third wrapping angle62. The number of layers may be any number, and the winding angles anddepth of each layer may likewise vary.

In another embodiment, resin mixture 36 (or the dry partially curedtowpreg 37) also comprises particles of a thermally conductive additive.The additive particulate may theoretically comprise any solid having ahigher thermal conductivity than the resin in the PMC, such as metal,ceramic, or chopped pitch carbon fiber. Graphene platelets, groundgraphite foam, or carbon nanotubes also have good thermal conductivity.Due to its combination of relatively low density, higher thermalconductivity, cost, and other superior attributes within the cured PMC,metal is a preferred thermal conductive additive material, and morepreferably aluminum.

As noted above, adding significant quantities of thermal conductingadditive adversely increases viscosity of resin mixture 36. For example,graphene platelets exhibit excellent thermal conductivity but tend tomake resin mixture unacceptably viscous. Graphene platelets might havean area (X-Y dimension) between 1 and 50 micrometers (μm) but athickness of only about 50-100 nanometers (nm), yielding an aspect ratioapproaching 1000:1. Particles having such high aspect ratios exacerbatethe viscosity issues afflicting polyimide resins discussed above. Theinventors have determined that rather than focusing on additivematerials having the best thermal conductivity, an alternate approach isto employ a material that allows maximization of additive volume versusadditive surface area. This approach suggests the additive particlesshould be approximately spherical.

In one embodiment, the additive particles are metal and have generallyspherical shape. The metal spheres comprise approximately 0.2% to 50% byweight of resin mixture 36 (about 0.1% to 25% by volume). In anotherembodiment, the additive particles are themselves comprised of two ormore sizes in order to more efficiently increase the thermalconductivity of the composite with minimal effect on processingcharacteristics. Having at least two sizes of thermally conductiveparticles in resin mixture 36 improves particle packing within theinterstitial spaces with less impact on the resin viscosity andconsequently improves heat transfer characteristics while keepingviscosity manageable.

FIG. 5 shows a greatly magnified cross sectional view of one embodimentof a cured polymer matrix composite 70. Polymer matrix composite 70could be produced either by wet winding resin or by winding partiallycured resin previously applied to towpreg. FIG. 5 shows cut ends ofindividual fiber strands/filaments 72 arranged generally parallel toeach other and surrounded by cured resin 74. Resin 74 occupies thespaces between fiber strands 72. The individual fiber strands aregenerally parallel and are approximately 3 to 15 microns (μm) indiameter. The interstitial spaces may range from about 1 micron to about50 microns. In one embodiment, a variety of sizes of thermal conductingadditive particles, preferably spherical metal particles, aredistributed in interstitial spaces within the resin 74 and fiber strands72 and in the interstitial spaces within resin 74.

In the embodiment shown, fiber strands 72 are approximately 7 μm indiameter and the thermal conducting additive comprises three sizes ofapproximately spherical aluminum particles, the smallest particles 76being about 0.1-1 μm in diameter, the medium particles 78 being about1-3 μm in diameter, and the large particles 80 being about 3-4 μm indiameter. These particle sizes can vary depending, for example, on thesize of the fibers. For example, the largest particles could measure 10μm. Most of the additive consists of small particles 76 and mediumparticles 78; a significantly smaller fraction is large particles 80. Byformulating and distributing the thermal conductive additive in suchfashion, many of the particles will be in close proximity or eventouching each other, and preferably in close proximity and/or touchingadjacent fiber tows 72, with the larger particles tending to occupy thelarger interstitial spaces and the smaller particles occupying thesmaller voids, which voids were formerly occupied by the solvent orvolatile fraction of resin mixture 36 that was volatilized in the curingprocess. The thermal conducting additive particles have higher thermalconductivity than resin 74, thereby making the PMC more thermallyconductive. On average, the plurality of sizes of thermally conductiveadditive spheres occupy a higher volume fraction of the interstitialspace otherwise present in the PMC, leading to higher thermalconductivity of outer shell 20.

After winding wet resin tow 42 or heated towpreg 43, the compositebarrel 14 is removed from the chucks 47 and subjected to heat and/orpressure to completely cure the thermoset polyimide resin. For wet resinsystems, depending on the amount of volatiles present prior tocommencing the cure process, a complete cure might require removingabout 15% of the mass of the freshly wound PMC structure. It isgenerally better to remove the volatiles earlier in the curing processto minimize formation of voids in the matrix.

Regardless of whether the tow is wound on inner liner 22 wet or dry, itis more difficult to cure structures incorporating polyimide resins thancommon epoxy-based resins. In wet resin applications particularly, it isdifficult to remove volatiles from the fiber resin matrix withoutcreating voids. When a polyimide resin is used with flat or large-radiuspanels, volatile transport is easier because volatiles can escape to theopen edges of the surface, and/or more readily migrate between fabriclayers. In filament winding applications, however, these gasses can betrapped between the continual windings.

Voids in the PMC have the undesirable effects of reducing strength,stiffness, and thermal conductivity. Satisfactory results are even moredifficult to achieve when curing an item produced by filament winding,in contrast to curing flat impregnated fabric sheets. The imperviousinner liner 22 forces volatiles to migrate radially outward through aplurality of densely wound layers (with a much smaller portion ofvolatiles migrating to the breech and muzzle 18 of barrel 14). Thecuring problem may be compounded still further when thermally conductiveadditives are present in resin mixture 36.

A cured outer shell 22 according to one embodiment is produced by firstproviding a freshly wound barrel incorporating PAN carbon fibers and aresin mixture 36 comprising P²SI 635LM polyimide resin and about 40%concentration by weight of generally spherical aluminum particlesbetween 1 and 5 microns. Such a resin mixture 36 has a glass transitiontemperature of about 635° F. after cure. The wet fiber tows 42 arepassed through a tube heater as described above then wound around aninner liner 22 using the wet-resin system depicted in FIG. 2 to generatea plurality of winding layers at a plurality of winding angles. Thefreshly wound barrel is then placed in an oven or autoclave and cured ina series of stages.

In a first stage, the temperature in the autoclave is gradually raised,over about 5 to 10 hours, to about 350° F. To assist in volatiletransport out of the PMC, vacuum may be applied to barrel 14 during thisstage. In a second stage, the oven or autoclave temperature is increasedto about 500-536° F. for between 2 and 8 hours to imidize the PMRpolyimide resin mixture solution to form oligomers having reactiveendcaps. At this stage, all volatiles re essentially removed from thecomposite shell and the functional endcapping agents on the oligomersmay start reacting and cross linking. During this second stage of thecure, pressure of between 10 and 400 psi, preferably about 200 psi, isapplied to facilitate consolidation. In a third stage, the temperaturewithin the oven or autoclave is raised even further to about 600-700°F., preferably for at least four hours, to accomplish a final cure,i.e., substantially completing cross-linking of the imidized polyimidesby reacting the endcapping agents and stabilizing the carbon-fiber/resinmixture matrix. The total curing time within the oven or autoclave ispreferably 14-24 hours. The autoclave may remain pressurized during thesecond and third stages.

Following cooling, the cured barrel 14 is placed on a lathe and grounddown to desired finish diameter with one or more abrasive tools such asdiamond-coated grinding and polishing wheels. For purposes of the claimsand this specification, “surrounding and in direct contact with theinner liner” means that outer shell 20 surrounds and is in directcontact with inner liner 22 along at least a portion of the axial lengthof barrel 14; parts of inner liner 22 may be exposed, for example, atmuzzle 18, threads 16, or any other desired location(s) on barrel 14.

The PMC embodiments described above include thermoset polyimide resinmixtures having glass transition temperature greater than 500° F.,allowing for hotter operation of barrel 14. Such resins demonstratesuperior thermal stability compared to epoxy-based resin mixtures, whichtypically undergo glass transition around 140-400° F. and haverelatively lower thermal stability. Not only is the structure of barrel14 better able to withstand higher temperatures, but an additionalbenefit is that the higher operating temperature facilitates heattransfer from metal liner 22 to the ambient atmosphere, due to a highertemperature difference between the external surface of outer shell 20and the atmosphere.

The foregoing invention has been described in accordance with therelevant legal standards, thus the description is exemplary rather thanlimiting in nature. Variations and modifications to the disclosedembodiment may become apparent to those skilled in the art and fallwithin the scope of the invention.

1. A barrel for directing the path of a dischargeable projectile,comprising: an inner liner defining an axial bore; and an outer shellsurrounding and in direct contact with the inner liner, said outer shellfabricated from a polymer matrix composite (PMC), said PMC comprisingfibers and a resin mixture, said resin mixture comprising a resin havinga glass transition temperature greater than 500° F.
 2. The barrel ofclaim 1 wherein said inner liner comprises a metal.
 3. The barrel ofclaim 2 wherein said metal comprises a steel alloy.
 4. The barrel ofclaim 1 wherein the fibers comprise carbon fiber.
 5. The barrel of claim4 wherein the carbon fiber comprises carbon fiber tows helically woundaround the inner liner in a plurality of layers.
 6. The barrel of claim5 wherein the plurality of layers include a plurality of winding anglesrelative to the axial bore.
 7. The barrel of claim 5 wherein at leastone of said plurality of layers comprises a plurality of PAN precursorcarbon fibers.
 8. The barrel of claim 7 wherein said PAN precursorcarbon fibers have a diameter of approximately 3 microns to 15 microns.9. The barrel of claim 5 wherein at least one of said plurality oflayers comprises a plurality of pitch precursor carbon fibers. 10.(canceled)
 11. The barrel of claim 1 wherein the resin is a thermosetresin.
 12. The barrel of claim 11 wherein the thermoset resin is apolyimide resin.
 13. The barrel of claim 1 wherein said resin mixturefurther comprises at least one thermally conductive additive.
 14. Thebarrel of claim 13 wherein said thermally conductive additive isselected from the group consisting essentially of: metal, ceramic,diamond, graphene, graphite, carbon nanotubes, and chopped pitch carbonfiber.
 15. The barrel of claim 13 wherein said thermally conductiveadditive comprises aluminum particles.
 16. The barrel of claim 15wherein said aluminum particles have a generally spherical shape. 17.The barrel of claim 16 wherein said aluminum particles comprise aplurality of sizes between about 0.1 microns and 10.0 microns indiameter.
 18. A barrel for directing the path of a dischargeableprojectile, comprising: an inner liner defining an axial bore; and anouter shell surrounding and in direct contact with the inner liner, saidouter shell fabricated from a polymer matrix composite (PMC), said PMCcomprising a resin mixture and fibers, said resin mixture comprisinggenerally spherical metallic particles, said metallic particlescomprising a plurality of sizes between about 0.1 microns and 10.0microns in diameter.
 19. The barrel of claim 18 wherein said metallicparticles are aluminum.
 20. A firearm comprising a receiver, a stockconnected to the receiver, and a barrel connected to the receiver,wherein the barrel comprises: an inner liner defining an axial bore; andan outer shell surrounding and in direct contact with the inner liner,said outer shell fabricated from a polymer matrix composite (PMC), saidPMC comprising a resin and fibers, said resin having a glass transitiontemperature greater than 500° F.
 21. A barrel for directing the path ofa dischargeable projectile, comprising: a steel alloy inner linerdefining an axial bore; and an polymer matrix composite (PMC) outershell surrounding and in direct contact with the inner liner, said PMCcomprising a thermoset polyimide resin having a glass transitiontemperature greater than 500° F., said PMC further comprising carbonfiber tows wound helically around the inner liner in a plurality oflayers and at a plurality of winding angles relative to the axial bore,said carbon fiber tows comprising a plurality of PAN precursor carbonfibers each carbon fiber having a diameter of about 3 to 15 microns,wherein generally spherical aluminum particles are dispersed throughoutthe PMC, said aluminum particles having a plurality of sizes betweenabout 0.1 microns and 10.0 microns in diameter.